The present invention relates to an FM signal demodulation system and in particular to threshold-extension demodulators for television systems.
It is a known practice to improve the noise C/N (carrier-to-noise) threshold characteristics in the demodulation of an FM signal over those obtainable with conventional frequency-discriminators, such as LC discriminators or delay-line discriminators, by using demodulators operating on a carrier-tracking principle. The prior-art class of carrier-tracking FM demodulators includes the phase-lock FM demodulator, the FM negative-feedback (FMFB) demodulator, and the variable-bandwidth dynamic-tracking (DTVB) demodulator.
In the FMFB type of demodulator, a negative-feedback control loop is utilized between the output of a limiter-discriminator and a voltage-controlled oscillator (VCO) input of a mixer of a downconverter of an FM receiving system, thereby effecting compression of the bandwidth of the input FM signal, the feedback being at video rate, with small loop delay, thus allowing an i.f. filter of narrower bandwidth than the bandwidth of the FM signal to be utilized. Inherent noise is thus deleted, thereby extending noise C/N (carrier-to-noise) threshold relative to the demodulation of the FM signal.
In the DTVB type of demodulator, an adaptive filter, having inputs for varying the center frequency of the filtering response of the adaptive filter, in response to a control-loop feedback signal, and for varying the bandwidth of the filter, in response to the strength of the FM signal, is utilized in conjunction with a limiter-discriminator; having an input coupled to be responsive to the FM signal and having an output coupled to an input of the limiter-discriminator. The feedback is utilized between the output of the limiter-discriminator and the input of the adaptive filter for varying the center frequency of the filter. The feedback is arranged, via appropriate scaling, offsetting and polarity selection of the output signal of the limiter-discriminator, to cause the center frequency, or the bandwidth window of the filtering response of the adaptive filter, to track, or substantially track, the instantaneous carrier frequency of the FM signal. The bandwidth of the adaptive filter is made less than the bandwidth of the FM signal, thereby deleting inherent noise and extending noise C/N threshold. The bandwidth of the adaptive filter is controlled by the bandwidth-controlling input according to the signal level detected by a detection means so that the bandwidth is narrow when the signal level is low, and the bandwidth is wide when the signal level is high. The detection means may be coupled to be responsive to the level of the signal or a component of the signal, such as the color subcarrier level.
However, prior-art carrier-tracking FM demodulators such as the above, for satellite television or other FM microwave video signals, have heretofore required critical feedback loops or difficult-to-design adaptive filters.